1. Field of the Invention
The present invention is generally related to proximity sensors and, more particularly, to a proximity sensor that is intended for detecting the open or closed status of a door and which utilizes a stationary portion that generates a pulsating magnetic field and a movable portion that is sensitive to that pulsating magnetic field.
2. Description of the Prior Art
Many different types of proximity sensors are known to those skilled in the art. One particular type of proximity sensor utilizes an eddy current killed oscillator, or ECKO, to detect the presence of a metal within a predefined detection zone. This type of proximity sensor is described below in conjunction with FIG. 1. One disadvantage in the use of certain proximity sensors, when they are applied for the purpose of detecting the closed status of a protective device, such as a door or a shield, is that they can be defeated by a tampering effort that places a metal object near the sensing coil of the proximity sensor for the purpose of simulating the door's closure. In other words, if someone desires to defeat the safety purposes of the proximity sensor and operate a machine with its protective door in an open position, this type of tampering effort can be attempted by placing a metal object next to the sensing coil of the proximity sensor with the protective door open so that the proximity sensor operates in the manner that it would operate if the door was closed. For this reason, many machine manufacturers are reluctant to incorporate proximity sensors as safety devices in the machines.
It would therefore be significantly beneficial if a proximity sensor could be provided in which a tampering attempt of this type could be detected and defeated.